The present invention relates to a reflection type diffuse hologram that can be used for display devices such as liquid crystal display devices, a hologram for reflection hologram color filters, etc., and a reflection type display device using such holograms.
Backlight used with a liquid crystal display device should have some scattering characteristics, so that the display device can have a wide viewing angle. So far, scattering characteristics have been imparted to backlight by use of beads or the like, but a problem with this is that too large an angle of diffusion results in wasteful illumination light loss.
This is also true of an automotive brake lamp or direction indicator. That is, although too large a diffusion angle is not required in view of the positional relation to succeeding cars, light from these lamps is not only wastefully consumed but also becomes dark because lenses positioned in front of the lamps cause the light to be diffused at an angle larger than required.
The present applicant has filed Japanese Patent Application No. 12170/1993 to come up with a color filter in which a hologram is used to achieve a remarkable increase in the efficiency of backlight used for liquid crystal display purposes, etc., and a liquid crystal display device that makes use of such a color filter.
A typical liquid crystal display device that makes use of this hologram color filter will now be briefly described with reference to a sectional view attached hereto as FIG. 43. As illustrated, a hologram array 55 forming the color filter is spaced away from the side of a liquid crystal display element 56 upon which backlight 53 is to strike, said element being regularly divided into liquid crystal cells 56′ (pixels). On the back side of the liquid crystal display element 56 and between the liquid crystal cells 56′ there are located black matrices 54. Although not illustrated, polarizing plates are arranged on the incident side of the hologram array 55, and the exit side of the liquid crystal display element 56. As is the case with a conventional color liquid crystal display device, between the black matrices 54 there may additionally be located an absorption type of color filters which transmit light rays of colors corresponding to pixels R, G, and B.
The hologram array 55 comprises micro-holograms 55′ which are arranged in an array form at the same pitch as that of R, G, and B spectral pixels, corresponding to the period of repetition of R, G, and B spectral pixels, i.e., sets of liquid crystal cells 56′, each including three adjoining liquid crystal cells 56′ of the liquid crystal display element 56 as viewed in a plane direction of the drawing sheet. One micro-hologram 55′ is located in line with each set of three adjoining liquid crystal cells 56′ of the liquid crystal display element 6 as viewed in the plane direction of the drawing sheet. The micro-holograms 55′ are then arranged in a Fresnel zone plate form such that a green component ray of the backlight 3 incident on the hologram array 55 at an angle θ with respect to its normal line is collected at a middle liquid crystal cell G of the three R, G, and B spectral pixels corresponding to each micro-hologram 55′. Each or the micro-hologram 55′ in this case is constructed from a relief, phase, amplitude or other transmission type of hologram which has little, if any, dependence of diffraction efficiency on wavelength. The wording “little, if any, dependence of diffraction efficiency on wavelength” used herein is understood to refer specifically to a hologram of the type which diffracts all wavelengths by one diffraction grating, much unlike a Lippmann type hologram which diffracts a particular wavelength alone but does not substantially permit other wavelengths to be transmitted therethrough. The diffraction grating having little dependence of diffraction efficiency on wavelength diffracts different wavelengths at different angles of diffraction.
In such an arrangement, consider now the incidence of the white backlight 53 from the side of the hologram array 55, which does not face the liquid crystal display element 56 at the angle θ with respect to its normal line. The angle of diffraction of the light by the micro-hologram 55′ varies depending on wavelength, so that light collection positions for wavelengths are dispersed in a direction substantially parallel with the surface of the hologram array 55. If the hologram array 55 is constructed and arranged such that the red wavelength component is diffractively collected at a red-representing liquid crystal cell R; the green wavelength component at a green-representing liquid crystal cell G; and the blue wavelength component at a blue-representing liquid crystal cell B, the color components pass through the corresponding liquid crystal cells 56′ with no or little attenuation through the black matrices 4, so that color displays can be presented depending on the state of the liquid crystal cells 56′ at the corresponding positions. It is here noted that the angle of incidence θ of backlight 53 on the hologram array 55 is determined by various conditions including hologram-recording conditions, the thickness of hologram array 55, and the distance between the hologram array 55 and the liquid crystal display element 56.
By using the hologram array 55 as a color filter in this way, the wavelength components of backlight used with a conventional color filter are allowed to strike on the liquid crystal cells 56′ without extravagant absorption, so that the efficiency of utilization thereof can be greatly improved.
The aforesaid hologram color filter proposed by the present applicant is applicable to only a color liquid crystal display device making use of backlight. However, when surrounding ambient light alone is used as illumination light, this hologram color filter cannot diffract, and collect its wavelength components into desired positions. In other words, this hologram color filter can never be applied to a direct-view type of liquid crystal display device or other like device in which surrounding ambient light is used as illumination light, or any particular backlight source is not required.
Moreover, the applicant has filed Japanese Patent Application No. 120016/1993 to come up with a method for using a swelling film to make from a volume hologram having uniform interference fringes recorded therein a color pattern that varies in reconstructed color depending on position. The principles are similar to those applied to a photo-polymer. First, a swelling film is prepared by mixing a monomer or oligomer, a photopolymerization initiator, etc. with a binder polymer. Then, the swelling film is irradiated with a given quantity of light before or after its close contact with a photopolymer or other photosensitive material having interference fringes recorded therein, so that a given proportion of the monomer or oligomer contained in the swelling film, on the one hand, is polymerized for deactivation and the amount of the remaining active monomer or oligomer, on the other hand, is controlled. The thus controlled amount of the monomer or oligomer is diffused, and swollen into the photosensitive material with interference fringes recorded therein, whereby fringe spacings are precisely controlled to any desired quantity to control reconstruction wavelengths to given ones. After this swelling treatment, the photosensitive material with the interference fringe recorded therein is irradiated with light or otherwise heated to fix the diffused monomer or oligomer in the interference fringes, so that there can be obtained a hologram excelling in the storage stability of reconstructed colors. In addition, a color pattern can be formed on the hologram by allowing the illumination light to have a spatial distribution.
This method will now be explained in a little more detail with reference to FIGS. 44 and 45. FIG. 44 illustrates the principles applied when the swelling agent (monomer or oligomer) contained in the swelling film is deactivated by irradiation with light after the swelling film has been brought into close contact with the photosensitive material, and FIG. 45 depicts the principles applied when the swelling agent contained in the swelling film is deactivated by irradiation with light before the swelling film is brought into close contact with the photosensitive material. Referring to FIG. 44(a), such a volume hologram 64 as depicted in FIG. 44(b) is obtained by striking object light 62 and reference light 63 on both sides of a photo-polymer or other photosensitive material 61 to record an interference fringe therein. As depicted in FIG. 44(c), a swelling film 65 prepared by mixing a monomer or oligomer, a photopolymerization initiator, etc. with a binder polymer is then brought into close contact with the photosensitive material. Subsequently, either the hologram 64 or the swelling film 65 is irradiated with light 66, as depicted in FIGS. 44(d1) to (d3), before or at the same time as heating is carried out to increase the degree of diffusion of the penetrating monomer or oligomer in the swelling film 65. This irradiation with light 66 causes a part or all of the penetrating active monomer or oligomer in the swelling film 65 to be polymerized, and so deactivated, at a proportion corresponding to the quantity of irradiating light 66, so that the ability of the monomer or oligomer to penetrate (diffuse) vanishes substantially. When the quantity of irradiating light 66 is large (FIG. 44(d1)), therefore, nearly all of the penetrating active monomer or oligomer in the swelling film 65 is deactivated, so that the monomer or oligomer does not substantially penetrate into the hologram 64 even upon being heated. If, for instance, interference fringes are recorded in the volume hologram 64 with a blue wavelength in FIG. 44(a), the hologram 64 subject to the swelling step in FIG. 44(d1) does not substantially swell, and diffracts and reconstructs blue light. When the quantity of irradiating light 66 is moderate (FIG. 44(d2)), on the other hand, about a half of the penetrating active monomer or oligomer in the swelling film 65 is deactivated. Another half of the penetrating monomer or oligomer penetrates into the hologram 64 upon being heated, which in turn swells moderately. For this reason, the hologram 64 subject to the swelling step shown in FIG. 44(d2) diffracts, and reconstructs green light that is longer in wavelength than blue light. Moreover, when the swelling film is not irradiated with light 66 (FIG. 44(d3)), nearly all of the penetrating monomer or oligomer from the swelling film 65 penetrates into the hologram 64, which in turn swells to the maximum extent. For this reason, the hologram 64 subject to the swelling step shown in FIG. 44(d3) diffracts, and reconstructs red light that is longer in wavelength than green light. By controlling the quantity of light 66 with which the swelling film 65 in close contact with the hologram 64 is irradiated, it is thus possible to optionally regulate the color to be reconstructed to one lying between red and blue.
Referring then to FIG. 45, especially FIGS. 45(a) and 45(b), a volume hologram 64 is obtained as depicted in FIGS. 44(a) and (b). As shown in FIGS. 45(c1) through (c3), a swelling film 65 is prepared by mixing a monomer or oligomer, a photo-polymerization initiator, etc. with a binder polymer. Upon this film being irradiated with a given quantity of light 66, a part or all of the penetrating active monomer or oligomer contained therein is deactivated at a proportion corresponding to the quantity of light 66, so that the ability of the monomer or oligomer to penetrate (diffuse) vanishes. When the swelling film 65 already irradiated with light 66 is brought into close contact with the hologram 64, as depicted in FIGS. 45(d1) through 45(d3), and then heated as shown in FIG. 44, the degree of swelling of the hologram 64 varies depending on the quantity of light 66. By controlling the quantity of light 66 with which the swelling film 65 is irradiated, it is thus possible to optionally regulate the color to be reconstructed to one lying between red and blue.
In this regard, it is noted that the swelling film 65 is prepared by mixing a monomer or oligomer, a photo-polymerization initiator, etc. with a binder polymer, and so is similar to a photopolymer used for recording holograms. Therefore, the hologram-recording photopolymer may be used as the swelling film 65; that is, it is unnecessary to prepare any special swelling film.
The aforesaid color pattern-making method proposed by the present applicant is to control the quantity of light with which the swelling film is irradiated before or after it is brought into close contact with a photosensitive material having interference fringes already recorded therein, thereby controlling the amount of the active monomer or oligomer contained in the swelling film, so that the proportion of swelling of the interference fringes (or the interference fringe spacings) can be controlled to regulate the color to be reconstructed to a given one. In short, the color to be reconstructed is controlled by the quantity of irradiating light.
However, one problem with the aforesaid method is that it is not always easy to precisely control the quantity of light to develop a given color, and another possible problem is that the reproducibility of the color reconstructed is not stable.